Floodplain Analysis
for
Tanuri Wash
Sections 24 and 25 of Township 13 South, Range 14 East, Section 19 and 30 of
Township 13, Range 15, Pima County, Arizona

By:
Pima County Regional Flood Control District
97 E Congress Street, 3rd Floor
Tucson, Arizona 85701
520-243-1800

June 2, 2008

Floodplain Analysis for Tanuri Wash
Prepared by
Pima County Regional Flood Control
97 E Congress, Tucson AZ 85701
June 2, 2008

Introduction
This study was prepared by Pima County Regional Flood Control District to provide
a drainage condition for Tanuri Wash. The site includes Sections 24 and 25 of Township
13 South, Range 14 East, Section 19 and 30 of Township 13, Range 15, Pima County,
Arizona. The study site is currently mapped as FEMA Zone A by approximate
methodology using USGS Quadrangle maps with 20 foot contours and no aerial
photography. The enclosed information would modify the floodplain limits for the
Tanure Wash as shown on Flood Insurance Rate Map (FIRM) number 04019C-1663K.

Site description
The study area is the downstream area of Tanuri Wash which is currently mapped as
Zone A. The entire watershed of Tanuri Wash is 1.8 sq miles with a mean slope of 0.025.
The watershed is mostly in a suburban foothills area covered with desert brush.

Methods
Hydrologic and hydraulic analyses were conducted using Pima County Hydrology
Procedures (PC-Hydro), Version 5.3.1, Hec-Ras 4.0 Beta version (HEC-RAS), HECGeoRAS, Version 4.1.1 (HEC-GeoRAS), and ArcGIS, Version 9.2.
A triangular Irregular Network (TIN) derived from Light Detection and Ranging
(LiDAR) data was used to create a topographic map. The locations of the stream
centerline, cross-sections, river banks, culverts, and other physical attributes of the wash
were determined by using the topographic map and aerial photos taken in 2005. The
physical attributes of the wash were digitized in ArcGIS using the HEC-GeoRAS
extension and then exported to HEC-RAS to create geospatially referenced geometric
data. Other parameters for the steady-state analysis, such as Manning’s n-values, culvert
data, expansion and contraction coefficients, normal depth boundary condition,
ineffective flow areas, and peak discharge rates obtained from PC-Hydro were manually
input into HEC-RAS. The hydraulic data obtained from HEC-RAS were then imported
into HEC-GeoRAS to delineate a floodplain in the study area.

Hydrologic analysis
The 100-year return interval peak discharge rates were computed at three
concentration points using PC-Hydro (Arroyo Engineering, 2007). NOAA Atlas 14,
upper 90% confidence interval, rainfall data were used for the analysis.
Hydrologic Soil Group B is the dominant soil type (98%). The Basin Factor used for
the hydrologic analysis ranges from 0.032 to 0.034. A vegetation cover density of 30%
was used to select the SCS Curve Number for the hydrologic calculation of all the
concentration points. Impervious cover percentage of 10% was determined using aerial
photos.
The 100-year peak discharge rates are 1884 cubic feet per second (cfs) at the
concentration point 1, 1092 cfs at the concentration point 2., and 2409 cfs at the
concentration point 3. The hydrologic computations are shown in Appendix 1.

Hydraulic analysis and floodplain delineation
Hydraulic analysis was performed in the area currently mapped as FEMA Zone A.
Steady flow analysis was performed to determine 100-year water surface elevations in the
study area by using HEC-RAS. As described above, geometric data for HEC-RAS
including stream centerline, cross-sections, river banks, and culvert were obtained by
using HEC-GeoRAS. The Hec-Ras data, TIN, aerial photos and shape files (lidar data,
contour lines, existing FEMA floodplain limit, section grid, street, study watersheds,
hydrologic soil groups, proposed floodplain limit) used in the analysis are included in an
attached CD.
Manning’s roughness coefficients for the main channel and the over-bank areas were
determined by using aerial photos taken in 2005. The roughness coefficient of 0.035 was
used for the main channel and 0.045 was used for the over-back areas.

Photo 1. Culvert at Tanuri Drive (left: upstream, right: downstream)

There are two box culverts installed for a road crossing at Tanuri Drive. Contraction
and expansion coefficients are 0.3 and 0.5 for immediately upstream and downstream of
the culvert, and 0.1 and 0.3 for other cross sections, which were obtained from HEC-RAS
Hydraulic Reference Manual. Normal depth with a slope of 0.02 was used as a boundary
condition for the steady flow analysis.
A floodplain map was created based on the hydraulic analysis. The map was created
using the coordinate system of 1983 HARN Stateplane Arizona Central, while the UTM
projection will be utilized for a final map. The two islands located between the cross
sections of St. 6424.835 and St. 6199.39, and St. 2921.178 and 2597.16 were removed
from the final work map. This is because the islands are considered to be unstable due to
the braded nature of Tanuri Wash. The final floodplain map is included in Appendix 2.
The floodplain map including the two islands is included in Appendix 3. The analysis
corrects several errors in the existing FEMA Zone A mapping. Current mapping shows
the floodplain extending over ridgelines and away from the actual channel. This analysis
shifts the FEMA Zone A floodplain toward the east north of the intersection of Wilmot
Rd. and Calle Vista Ciudad. The corrected floodplain is shifted toward the west south of
River Road. Additional information provided in this analysis shows hazardous conditions
exist for motor travel at Tanuri Drive which is inundated by approximately 2-feet during
the 100-year flood event.
References
HEC-RAS Hydraulic Reference Manual version 3.1, US Army Corps of Engineering
Center, November 2002.
PC-Hydro User Guide, PC-Hydro V5 Pima County Hydrology Procedures, A computer
program for predicting peak discharge of surface runoff from small semi-arid watersheds
in Pima County, Arizona, Arroyo Engineering, LLC, 2007

Appendix 1

Appendix 2

Appendix 3